1. Field of the Invention
This invention relates to a method and an apparatus for growing aluminum nitride (AlN) single crystal boules. More particularly, AlN single crystals are grown in a crucible made from a refractory metal (tantalum or niobium) or metal carbide (tantalum carbide or niobium carbide), or metal nitride (tantalum nitride or niobium nitride) or a composite of refractory metal, metal carbide, and metal nitride. The crucible portion in contact with the AlN crystal boule growing inside the crucible has a thickness nominally in the range of 0.05-2.0 mm and preferably in the range of 0.1-1.0 mm. In one aspect of the invention, the crucible is heated inductively or resistively. The induction susceptor (heat receiver), in the case of an inductively heated furnace, or a heater, in the case of a resistively heated furnace, and the thermal insulation are made from graphite-based or non-graphite-based materials. AlN single crystal boules are grown in the growth setup by sublimation physical vapor transport at an elevated temperature, nominally in the range of 2000-2500° C., more preferably in the range of 2150-2450° C. In other embodiments, the pressure inside the growth vessel during crystal growth is maintained by filling or flowing nitrogen and argon gases and the partial pressures of nitrogen and argon gases are maintained nominally in the ranges of 300 to 2000 torr and 0 to 800 torr, respectively, and a total system pressure maintained in the range of 300 torr to 2800 torr.
2. Description of the Related Art
Concepts related to AlN sublimation physical vapor transport (referred to as PVT hereinafter) growth are briefly reviewed. A sublimation physical vapor transport growth technique for AlN crystals is essentially a sublimation and re-condensation process, in which an AlN source material placed in a crucible (container) is sublimed into a mixture of nitrogen (N2) gas and aluminum (Al) vapor at high temperatures, usually higher than 1800° C., and the vapor species then diffuses to the cooler end of the crucible to recombine and form AlN crystals. If the vapor phase resulting from sublimation of AlN solids has a nitrogen to aluminum molar ratio, i.e. the number of N2 molecules to the number of Al molecules, of exactly 1 to 2 (i.e. 0.5), the vapor phase is stoichiometric. When the ideal gas law is applied, the ratio of N2 partial pressure to Al vapor partial pressure in a stoichiometric vapor, and the corresponding partial pressures of the nitrogen gas and the aluminum vapor are called stoichiometric partial pressures. AlN sublimed in a sealed and gas-tight container with no prior nitrogen gas or aluminum metal placed in it produces a stoichiometric vapor. The vapor phase can also be made to deviate from stoichiometry. The vapor phase is called nitrogen-rich vapor when the nitrogen gas to aluminum molar ratio is larger than 1 to 2, and the vapor phase is called aluminum-rich when the nitrogen gas to aluminum molar ratio is smaller than 1 to 2.
PVT AlN growth can be carried out in a crucible that is capable of communicating with the environment outside of the crucible, and examples of communicating crucibles include a substantially open crucible allowing nitrogen gas and aluminum vapor to go in or out the crucible, and a substantially sealed crucible preventing excessive loss of aluminum vapor and allowing nitrogen to diffuse through the seal to the extent that the total pressure inside and outside the crucible is substantially the same. Using a communicating crucible in a PVT growth is advantageous, compared to a sealed crucible, because, by controlling the system nitrogen pressure inside the furnace, the stoichiometry of the vapor phase can be set or controlled to any desired stoichiometry, whether it is stoichiometric, or nitrogen-rich, or aluminum-rich, in-situ during a growth. More specifically, at a given source material temperature in a PVT growth, the system nitrogen pressure can be set to a value so that the nitrogen partial pressure inside the growth crucible equals the stoichiometric nitrogen partial pressure so that a substantially stoichiometric vapor is obtained. Similarly, at a given source material temperature in a PVT growth, the system nitrogen pressure can also be set to a value so that the nitrogen partial pressure inside the growth crucible is higher than the stoichiometric nitrogen partial pressure so that a nitrogen-rich vapor is obtained. Moreover, at a given source material temperature in a PVT growth, the system nitrogen pressure can also be set to a value so that the nitrogen partial pressure inside the growth crucible is lower than the stoichiometric nitrogen partial pressure so that an aluminum-rich vapor is obtained.
In a PVT AlN growth, adding inert gas, such as argon, into the crucible will not change the nitrogen to aluminum molar ratio in the vapor phase, but may slow down diffusion transport of the vapor species and hence may reduce growth rate. Adding argon gas into a PVT growth furnace may significantly decrease degradation of PVT furnace parts, including heater and thermal insulation, exposing to high temperatures during PVT growths and therefore increase their useful lifetime.
Growth of bulk AlN single crystal boules using a sublimation physical vapor transport technique was demonstrated by Slack & McNelly (G. A. Slack and T. McNelly, “Growth of High Purity AlN Crystals”, J. Cryst. Growth, 34 (1976), and G. A. Slack and T. McNelly, “AlN Single Crystals”, J. Cryst. Growth, 42 (1977), referred to as Slack's work hereinafter). Slack's work teaches a use of thin wall tungsten crucibles. Slack's work yielded polycrystalline AlN crystals but the crystal growth rates were rather low, usually about 0.3 mm per hour. Slack's work also disclosed that the thin wall tungsten crucibles used in the PVT growth experiments suffered from leakage of aluminum vapor leading to crucible failure during growth, which limited AlN crystal boules to small usable lengths (usually less than 10 mm).
For volume production of AlN crystals, a growth rate higher than 0.3 mm/hr in PVT AlN growth is desirable because the higher the growth rate is, the more productive and economical the boule growth process is. A longer lifetime of crucibles is highly desirable because it permits growth of longer boules yielding more crystal substrates per boule. To develop a reliable and efficient physical vapor transport growth technique for volume production of AlN single crystal substrates, a number of researchers and artisans made attempts to improve PVT growth technique for AlN crystals over the technique disclosed in Slack's work.
Growth rate in sublimation PVT growth of AlN was modeled and studied theoretically by Dryburgh (P. M. Dryburgh, “The Estimate of Maximum Growth Rate for Aluminum Nitride Crystals Grown by Direct Sublimation”, J. Cryst. Growth, 125 (1992), referred to as Dryburgh's model). Dryburgh's model deals with a sublimation growth from a stoichiometric vapor. Dryburgh's model predicts that the growth rate of AlN crystal increases as the crystal temperature increases when the temperature difference between the source material and growing crystal, and the system nitrogen pressure are held constant.
Segal, et al., (A. S. Segal, S. Yu. Karpov, Yu. N. Makarov, E. N. Mokhov, A. D. Roenkov, M. G. Ramm, and Yu. A. Vodakov, “On Mechanisms of Sublimation Growth of AlN Bulk Crystals”, J. Cryst. Growth, 211 (2000), referred to as Segal's work, hereinafter) studied PVT AlN growth both theoretically and experimentally. Segal's work further deals with PVT AlN growth from both a stoichiometric vapor phase and a non-stoichiometric vapor phase. Segal's work further predicts that for a given set of temperatures of the source material and the growing crystal, the crystal growth rate depends on the stoichiometry of the vapor phase in the following manner: the growth rate reaches its maximum when the vapor phase is close to stoichiometric, and the growth rate decreases as the nitrogen partial pressure increases above the stoichiometric nitrogen partial pressure, i.e. from a nitrogen-rich vapor, and the growth rate also decreases when the nitrogen partial pressure in the vapor phase is lower than the stoichiometric nitrogen partial pressure, i.e. from an aluminum-rich vapor. Segal's work teaches use of an open crucible so that a wide range of nitrogen-to-aluminum molecular ratios in the vapor phase inside the crucible can be achieved by controlling the system nitrogen pressure exterior to the crucible. However, compared to a sealed crucible used in Slack's work, use of an open crucible has two major disadvantages: (1) a large amount of aluminum vapor is wasted, and (2) the excess aluminum vapor escaped from the crucible would reacts and degrades or even destroys the heater/susceptor and thermal insulation in the growth furnace.
Hunter's patents (U.S. Pat. Nos. 5,858,086; 5,972,109; 6,045,612; 6,063,185; 6,086,672; and 6,296,956) describe various growth setup schemes for PVT AlN growth. The patents disclose crucibles made from graphite, silicon carbide coated graphite, aluminum oxide, zirconium oxide and boron nitride. Crucibles made from these materials are typically rendered unusable at growth temperatures higher than about 2000° C., due to severe chemical reaction of these crucible materials with aluminum vapor or because of the low melting points of some of these crucible materials.
U.S. Pat. No. 6,770,135 to Scholwater, et al., discloses a growth apparatus and a growth method for AlN single crystal boule growth with: (i) a substantially sealed crucible made of metallurgical tungsten, (ii) a thermal insulation made of a combination of tungsten and BN (or other non-graphite-based thermal insulation materials), (iii) a gas mixture of H2 (no more than 5%), N2 and Ar gases at a super-atmospheric pressure (>1 atm, or 760 torr) inside the growth vessel and the crucible, and (iv) the crystal growth initiated and maintained by traversing the crucible with respect to the heater and thermal insulation.
Although the use of a substantially sealed tungsten crucible preventing excessive leakage of aluminum vapor from the crucible leads to a better usage of the AlN source material and a higher growth rate, the crystal boules are grown inside a tungsten crucible in a manner that the crystal boules adhere to the walls of the tungsten crucible. This poses at least two problems: (1) difficult crystal boule retrieval—because the AlN crystal boules adhere to the walls of the tungsten crucibles, retrieval of the AlN crystal boules from the crucibles is difficult and may also cause stresses and even cracks in the crystal boules; and (2) high thermal-mechanical stresses in the crystals—because the thermal expansion coefficient of metallurgical tungsten metal is smaller than AlN crystals, the grown AlN crystal boules cooled down from growth temperature to the room temperature will be under a tensile stress, which will cause stresses and even cracks in the AlN crystals boules. These problems may be partially alleviated if a thin wall tungsten crucible is used and the wall thickness in the crucible portion adhering to the AlN crystal boule is significantly smaller than 2 mm. Crucibles made from metallurgical tungsten (the most commonly produced tungsten metal) are extremely brittle, and hence machining, or otherwise making, a tungsten crucible with a wall thickness significantly less than about 2 mm is difficult. Furthermore, a tungsten crucible of a regular wall thickness (about 2 mm or larger) is already prone to cracking arising from the “swelling effect” due to infiltration of aluminum vapor into the tungsten crucible from the inside surfaces of the tungsten crucibles in contact with the vapor phase during a PVT AlN growth, as disclosed in US Patent Application Pub. No. US 2003/0127044, Schowalter, et al.), and a tungsten crucible with a thin wall significantly less than about 2 mm will have too short a lifetime to produce AlN crystals of useful lengths, as found in Slack's work.
In Schowater's patent, since growth initiation and continuation are achieved by traversing the crucible relative to the thermal gradient, control of crystal growth rate is complicated by a constant change of thermal profile within the growth crucible and the variation of the growth rate within a growth run may affect the quality of the crystal boules. In addition, the constant change of thermal profile inside the crucible can cause a constant change of the shape of the interface between a growing crystal boule and the adjacent vapor phase, and such a change of growth interface shape from convex (towards the vapor phase) to concave can cause unfavorable nucleation and growth on the crucible wall leading to a defective crystal boule or a polycrystalline boule.
Epelbaum, et al., (B. M. Epelbaum, M. Bickermann and A. Winnacker, “Sublimation Growth of Bulk AlN Crystals: Process Temperature and Growth Rate”, Materials Science Forum, Vols. 457-460 (2004), referred to as Epelbaum's work hereinafter) teaches open tungsten crucibles heated by a tungsten resistive heater in a PVT AlN growth process. The open crucibles used in Epelbaum's work have the same disadvantages as in Segal's work. Epelbaum's work demonstrated a growth rate of 0.3-1.0 mm/hr, but the poor integrity of the tungsten crucibles at high growth temperatures (>2150° C.), presumably due to the “swelling effect”, was also found to limit the crystal boule length.
Zhuang, et al., (Dejin Zhuang, Raoul Schlesser and Zlatko Sitar), “Crystal Expansion and Subsequent Seeded Growth of AlN Single Crystals”, Mater. Res. Soc. Symp. Vol. 831(2005); and (D. Zhuang, Z. G. Herro, R. Schlesser, and Z. Sitar), “Seeded Growth of AlN Single Crystals by Physical Vapor Transport”, J. Cryst. Growth 287 (2006), referred to hereafter as Zhuang's work, disclose an open crucible made of tungsten in a graphite-based susceptor and thermal insulation system within an inductively heated furnace. Disadvantageously, the growth technique in Zhuang's work suffers from the same drawbacks of open crucibles made of tungsten metal.
U.S. Pat. No. 7,056,383, referred to as Heleva's patent hereinafter, discloses a use of tantalum nitride (TaN)-based crucibles converted from a tantalum crucible in a graphite-based or a non-graphite based PVT furnace for AlN crystal growth. Disadvantageously, Heleva's patent teaches a use of open crucibles in PVT AlN growth that suffers from the same drawbacks as previously described in growths using open crucibles made of tungsten.
Therefore, in order to efficiently produce high quality AlN single crystals suitable for making substrates for group III-nitride device fabrication, there remains a need of a PVT AlN crystal growth technique that overcomes the deficiencies in the prior art.